Hypnosis is a human condition involving focused attention (the selective attention/selective inattention hypothesis, SASI), reduced peripheral awareness, and an enhanced capacity to respond to suggestion.There are competing theories explaining hypnosis and related phenomena. Altered state theories see hypnosis as an altered state of mind or trance, marked by a level of awareness different from the ordinary state of consciousness. In contrast, non-state theories see hypnosis as, variously, a type of placebo effect, a redefinition of an interaction with a therapist or a form of imaginative role enactment.During hypnosis, a person is said to have heightened focus and concentration and an increased response to suggestions.

Hypnosis usually begins with a hypnotic induction involving a series of preliminary instructions and suggestions. The use of hypnosis for therapeutic purposes is referred to as "hypnotherapy", while its use as a form of entertainment for an audience is known as "stage hypnosis," a form of mentalism.

Hypnosis-based therapies for the management of irritable bowel syndrome and menopause are supported by evidence. Use of hypnosis for treatment of other problems has produced mixed results, such as with smoking cessation. The use of hypnosis as a form of therapy to retrieve and integrate early trauma is controversial within the scientific mainstream. Research indicates that hypnotising an individual may aid the formation of false memories, and that hypnosis "does not help people recall events more accurately".

James Braid (19 June 1795 – 25 March 1860) was a Scottish surgeon, natural philosopher, and "gentleman scientist".

He was a significant innovator in the treatment of clubfoot, spinal curvature, knock-knees, bandy legs, and squint; a significant pioneer of hypnotism and hypnotherapy, and an important and influential pioneer in the adoption of both hypnotic anaesthesia and chemical anaesthesia. He is regarded by some, such as Kroger (2008, p. 3), as the "Father of Modern Hypnotism"; however, in relation to the issue of there being significant connections between Braid's "hypnotism" and "modern hypnotism" (as practised), let alone "identity", Weitzenhoffer (2000, p. 3) urges the utmost caution in making any such assumption:

It has been a basic assumption of modern (i.e., twentieth century) hypnotism that it is founded on the same phenomenology it historically evolved from. Such differences as exist between older versions of hypnotism and newer ones being reduced largely to a matter of interpretation of the facts. That there are common elements is not in question, but that there is full identity in questionable and basically untestable. – Weitzenhoffer (2000, p. 3; emphasis added).

Also, in relation to the clinical application of "hypnotism",

Although Braid believed that hypnotic suggestion was a valuable remedy in functional nervous disorders, he did not regard it as a rival to other forms of treatment, nor wish in any way to separate its practice from that of medicine in general. He held that whoever talked of a "universal remedy" was either a fool or a knave: similar diseases often arose from opposite pathological conditions, and the treatment ought to be varied accordingly. – John Milne Bramwell (1910)